Variable speed drive for spiral scanner



Aug. 18, 1959 N SOREL ETAL 2,900,519

VARIABLE SPEED DRIVE FOR SPIRAL SCANNER Filed March 51, 1954 5sheets-sheet 1 VHB/HELE SPEED DEI VE IN VEN TOR. NHT/709A! 5026/.

Aug. 18, 1959 N. SOREL ET AL VARIABLE SPEED DRIVE EOE sPIRAL SCANNERFiled'marcn s1, 1954 3 Sheets-Shet 2 l mafw VEW Y mw.. E 6% N E A wmNwv.

Aug. 18, 1959 N. SOREL ET AL A 2,900,519

VARIABLE SPEEDDRIVE FOR SPIRAL SCANNER Filed March 31,A 1954 3Sheets-Sheet 3 IZEL :77.j

5 zooo (500m/NEE RPM United States Patent VARIABLE SPEED DRIVE EoRsPnzAL SCANNER Nathan Sorel, Carle Place, and Warren A. Wiener, Islip,N.Y., assignors to the United States of America as represented by theSecretary of the Air Force Application March 31, 1954, Serial No.420,214

2 Claims. (Cl. Z50-83.3)

This invention relates to spiral scanning systems of the type in whichthe scanning beam generates a cone of continuously varying apex angle.In such scanning systems the scanning head, which produces the scanningbeam, is rotated about an axis fixed relative to the vscanner and at thesame time the tilt angle of the beam rela- .tive to the axis of rotationis varied back and forth between zero and a predetermined maximum value.This results in a spiral scanning pattern in planes perpendicular to theaxis of rotation..

It is evident that the writing speed or tangential velocity of the beamin a spiral scan varies widely from low values of tilt angle to highvalues if the beam is rotated at constant speed. For example, if themaximum tilt angle is 10, the writing speed at 10 is approximately tentimes that at 1. In target seekers of the heat sensitive type theamplitude and duration of the signal produced-when the beam intercepts atarget are directly related to the dwell .time of the target on thebolometer. Accordingly, in the above example, a target at 1 tilt woulddwell on the bolometer element approximately ten times as long as atarget at l0". For uniform sensitivity the dwell should be equal overthe entire scanned field. Ac-

cordingly, it is the object of this invention to provide means forproducing uniform beam writing speed in a spiral scan. This is an idealsituation, however, and is diicult -to achieve due to the high inertiaof the rotating mass and the resulting high power required to effectVthe necessary acceleration during one scanning frame. As a practicalmatter, therefore, the dwell time is made equal to the bolometer timeconstant at mid-iield and the tangential speed range reduced to about3:1.

Briefly, this is accomplished by a variable speed drive for the scannerwhich is controlled by the tilt mechanism.

The control is such that the scanner has its maximum rotational speedwhen the beam tilt angle is zero and its minimum rotational speed whenthe beam tilt angle is at its maximum value.

A more detailed description of the invention will be given in connectionwith the specific embodiment thereof shown in the accompanying drawings,in which- -Fig..1 isa block diagram of a spiral scanning mechanism withvariable speed drive in accordance with the invention;

j Fig. 2 shows the details of the Variable speed drive; and y Fig. 3shows speed control characteristics for uniform writing speed andawriting'speed range of about 3:1.

Referring .to Fig. l, which'shows the general applica- Y tion `oftheinvention to a spiral scanner for use in an infra-redY 'sensitive''target seeker, the scanning head 1 contains 'a mirror '2 "and a'bolometer 3 positioned in front of the mirror to receive reections ofinfra-red energy from any source thereof intercepted by the scanningbeam 4.v The mirror is rotated by shaft 5 and at the same time the angleg between its optical axis and the axis of rotation is varied cyclicallyfrom zero to a maximum value, for example and back to zero by tiltmechanism 6.

The sum of these movements causes the beam, which a1- ways rotates inthe same direction, to describe inward and outward spirals in a planeperpendicular to the axis of rotation. An inward spiral is shown inFig. 1. The bolometer may either be attached to the mirror andpositioned at its focal point, in which case the mirror tilt angleequals the beam tilt angle g, or the bolometer may be supported on theframe of the scanner in a fixed position, in which case the mirror tiltangle equals one-half the beam tilt angle g. The latter method resultsin less' uniform sensitivity over the scanned eld but avoids thebolometer microphonics due to attaching the bolometer to the rotated andtilted mirror. The output of the bolometer is applied over conductor 7to .the target information processing portion of the seeker, not shown.

Shaft 5, which rotates mirror 2, is driven by motor 8 through variablespeed drive 9.. Tilt mechanism 6 comprises a lead screw 10 which isdriven from shaft 5 through suitable gearing and reversing clutch 11.Tilting of the beam is accomplished through arm 12 by rotation of leadscrew 10 in either direction. Assuming the beam to be spirally inwardlyas shown, limit switch S1 closes when g=0 and, acting through reversingclutch control 1-3 and reversing clutch 11, reverses the direction ofrotation of lead screw 10. The beam then spirals outwardly until greaches its maximum value, in this case 10, at

which point limit switch S2 closes again reversing leadv screw 10 andinitiating the next inward spiral.

As already stated, if the mirror is rotated at constantv speed, thewriting speed or tangential velocity of the beam is approximatelyproportional to g, which, in the example shown, means about a 10:1variation in writing speed and target dwell over the scanned area. If Vis the writing speed, and A the revolutions-perminute of the mirror, theexact expression for the range of writing speeds is V max tan g max Amin V min tan g min A max since the tangent of an angle not greater than10 is substantially proportional to the angle this ratio issubstantially equal to g max A min value the mirror 2 is driven from thevariable speed, drive 9 which is controlled by the tilt mechanismthrough actuation of control potentiometer 14. The control is such thatshaft 5 has its maximum rotational speed when g is zero and its minimumrotational speed when g has its maximum value, in this case 10.

' The details of the variable speed drive are shown in Fig. 2. Speedcontrol of shaft 5 is accomplished through the action of differentialgear 15 and magnetic brakes v16 and 17. `The spider of the differential,which carries pinions 18 and 19, is driven at constant speed by motors..

through gear 20, Magnetic-brake 16 is connected to output gear 21through shaft 22, While magnetic brake 17 is connected to output gear 23by kshaft 24 which passesfreely through gear 20. YOutput shaftS isdriven ,at cthe same speed as shaft 24 by 1:1 gears 25-'26. The'magnetic brakes are designed to be applied when their coils areenergized. If brake 16 is applied and brake 17 released,

`output shaft 5 rotates at twice the speed of spider gear 20, whereas,if brake 17 is applied and brake 16 released, the output shaft speed iszero. Therefore by alternate application of brakes 16 and 17 the speedof shaft 5 may Patented Aug. 18, 1959,

The currents through thecoils of brakes 16 and 17 are controlled bytubes 27 and 28, respectively, whichA are in turn controlled by adifferential amplifier comprising tubes 29 and 30. A potentiometer31-32is providedY for applying a positive reference voltage to the grid oftube 30. A Voltage e isfapplied to the grid of tube 29 that is directlyrelated vto the speed of shaft 5 and the beam tilt angle g. This voltageis derived from D.C. tachometer 33, which has a speed and output voltageproportional to the speed of shaft S, and a voltage divider consistingof speed control potentiometer 14 and resistors 34 and 35. The amount ofvoltage division is controlled by tap 36 which is actuated by the tiltmechamsm.

The operation of the variable speed control is as follows: Assume thatcontact 36 is fixed, that e=E and that contact 37 has been adjusted tosuch position that the equal currents through tubes 27 and 28 are justbelow the operating thresholdof brakes 16 and 17. For this conditionbrakes 16 and 17 are released and the load on shaft tends to reduce itsspeed. Any reduction in the speed of shaft 5 however reduces the outputof D.C. tachometer 33 and therefore reduces e. A reduction in e reducesthe bias on tube 27 from resistor 38 and causes the currentrin this tubeto increase and apply brake 16. The decrease in e has no effect on brake17 since this decrease acts through resistor 39 to increase thepotential Vof the grid of tube 30 relative to its cathode whichincreases the current through resistor 40 and, as a result, increasesthe bias on tube 28, so that the current through the coil of brake 17 isreduced still further below its threshold value. the speed of shaft 5and consequently e to increase. The increase kin e reduces the currentin tube 27 and, acting through resistor 39, increases the current intube 28 until brake 16 is released and brake 17 is applied. This actionslows shaft 5 and tends to again bring e into equality with E.Therefore, for any position of contact 36, the control circuit tends tokeep e Within a small range centered about E and the speed of shaft 5within a small range centered about a speed determined by the value of Eand the relative values of resistors 35 and 14-34. The extent of thisrange of variation is determined by the sensitivity of the controlcircuit, i.e., by the gain of the amplifiers.

If the position of contact 36 is changed the speed of shaft 5 changesaccordingly. Assume that the tilt mechanism is operating in the outwarddirection so that g is increasing. This results in contact 36 beingmoved to the right in Fig. 2 and e to increase. The increase in ereduces the current in tube 27 and increases the current in tube 28, inthe manner explained above, so that brake 16 is released and brake 17applied thus reducing theV speed of shaft 5 until e again equals E. Thisequality will now be attained at a lower speed of shaft 5 since e nowconstitutes a greater portion of the output voltage of the D.C.tachometer than formerly. Conversely, if contact 36 is moved in the ndirection the portion of the tachometer output voltage represented by eis decreased and consequently a higher speed of shaft 5 is required tobring e into equality with E. Movement of contactA 36, therefore, variesthe speed of shaft 5 from a maximum value at the 0 end of potentiometer14 to a minimum value at the end.

By suitable choice of reference voltage E and the dividing 'ratio of thevoltage divider made up of resistors 35 and 14-34, various speed controlcharacteristics may be obtained. In general, voltage E determines thespeed The braking of gear 21 causesY compromise between writing speedvariation, power re quirement and search frame period. For a beam widthof 1 and values of g not exceeding 10 the dwell time is substantiallyequal to 60,000 (4) -v 2mm vmilliseconds where A is in r.p.m. and g indegrees. For highest sensitivity the dwell time should be close to orexceed the bolometer timerconstant which is of the order of 2.5milliseconds. Curve B of Fig 3 illustrates a compromise design in whichthe dwell time at g=5 is 2.5 millisecf onds, falling to 1.9 millisecondsat g=l0 and rising to 7.8 milliseconds at g=1. A max is 1560 r.p.m. atg=0 and 500 r.p.m. at g=l0 for a speed range of 3.1:1. The range ofwriting speeds is therefore 3.2:l. Curve A illustrates the speed controlcharacteristic required for a uniform dwell time of 2.5 milliseconds.

4We claim:

l. A spiral scanner comprising means forming a scanning beam, variablespeed rotatingv means for rotating said scanning beam about an axispassing through said beam forming means and fixed relative to saidscanner, means associated with said beam forming means for cyclicallyvarying the tilt angle of said beam relative to said axis from a minimumvalue to a maximum value and back to said minimum value, meansassociated with said rotating means and said tilt angle varying meansfor establishing a voltage that is a direct function of both the speedof rotation of said'beam and the tilt angle of said beam, and meansresponsive to said voltage and actingon said rotating means formaintaining said voltage constant by controlling the speed of rotationof said beam.

2. A spiral scanner comprising means forming a scanning beam, means forrotating said scanning beam about an axis passing through. said beamforming means and fixed relative to said scanner, means associated withsaid' beam forming means for cyclically varying the tilt angle of saidbeam relative to said axis from a minimum value to a maximum value andback to said minimum value, a dierential gear, means for driving thespider of said differential gear at constant speed, means for brakingone output gear of said differential gear, means for braking Y the otheroutput gear of said differential gear, means for coupling said oneoutput gear to said beam rotating means, means associated with said beamrotating means and said tilt angle varying means for generating avoltage that is a direct function of both the speed of rotation of saidbeam and the tilt angle of said beam, and means responsive to saidvoltage and acting on said braking means for applying thebraking meansof said one output gear when said voltage exceeds a predetermined valuegear when said voltage falls below said predetermined value.

References Cited in the file of this patent UNITED STATES PATENTS

